Samlex America PST-200S-24E - Manual (Page 22)

22 | SAMLEX AMERICA INC.
In the example given above, the 10.7V Low Battery / DC Input Alarm would trigger at
around 80% discharged state (20% SoC) when the C-Rate discharge current is C/5 Amps.
However, for lower C-Rate discharge current of C/10 Amps and lower, the battery will be
almost completely discharged when the alarm is sounded. Hence, if the C-Rate discharge
current is lower than C/5 Amps, the battery may have completely discharged by the time
the Low DC Input Alarm is sounded.
Low Dc input Voltage Shut-down in inverters: As explained above, at around 80% State
of Discharge of the battery at C-Rate discharge current of around C/5 Amps, the Low
DC Input Voltage Alarm is sounded at around 10.7V for a 12V battery (at around 21.4V
for 24V battery) to warn the user to disconnect the battery to prevent further draining
of the battery. If the load is not disconnected at this stage, the batteries will be drained
further to a lower voltage and to a completely discharged condition that is harmful for
the battery and for the inverter.
Inverters are normally provided with a protection to shut down the output of the
inverter if the DC voltage at the input terminals of the inverter drops below a threshold
of around 10V for a 12V battery (20V for 24V battery). Referring to the Discharge Curves
given at fig 7.1, the State of Discharge for various C-Rate discharge currents for battery
voltage of 10V is as follows: (Please note that the X-Axis of the curves shows the % of
State of Charge. The % of State of Discharge will be 100% - % State of Charge):
- 85% State of Discharge (15% State of Charge) at very high C-rate discharge
current of C/3 Amps.
- 100% State of Discharge (0 % State of Charge) at high C-Rate discharge
current of C/5 Amps.
- 100% discharged (0% State of charge) at lower C-rate Discharge current
of C/10 Amps.
It is seen that at DC input voltage of 10V, the battery is completely discharged for C-rate
discharge current of C/5 and lower.
In view of the above, it may be seen that a fixed Low DC Input Voltage Alarm is not use-
ful. Temperature of the battery further complicates the situation. All the above analysis
is based on battery electrolyte temperature of 78°f. The battery capacity varies with
temperature. battery capacity is also a function of age and charging history. older bat-
teries have lower capacity because of shedding of active materials, sulfation, corrosion,
increasing number of charge / discharge cycles etc. Hence, the State of Discharge of a
battery under load cannot be estimated accurately. However, the Low DC Input Voltage
Alarm and Shut Down function are designed to protect te inverter from excessive cur-
rent draw at the lower voltage.
Use of external Programmable Low Voltage Disconnects
The above ambiguity can be removed by using an external, programmable Low Voltage
Disconnect where more exact voltage threshold can be set to disconnect the battery
seCtIOn 7 | General Information on Batteries for
Powering Inverters